Recently, the pressure-sensitive adhesive sheets have been employed in various aspects such as fixation (bonding), conveyance, protection, decoration, and the like of articles. Typical examples of the pressure-sensitive adhesive sheets include those provided with a pressure-sensitive adhesive layer formed by using an acrylic adhesive composition. This acrylic adhesive composition is configured such that it may form a pressure-sensitive adhesive layer containing an acrylic copolymer as a base polymer. As the acrylic copolymer, those containing alkyl (meth)acrylate as a main ingredient (a main component) and further containing a copolymerization composition containing a carboxyl group-containing monomers such as acrylic acid, for the purpose of improving adhesive performance, and the like are generally used.
However, for a pressure-sensitive adhesive sheet used for the articles having a part composed of a material that is susceptible to an acid (for example, an article having a metal surface, such as electronic parts), when it has the copolymerization composition containing a carboxyl group-containing monomer as described above, the carboxyl group in the acrylic copolymer may be responsible for the unfavorable effects on the above-described material (for example, the corrosion of the metal surface). Accordingly, the pressure-sensitive adhesive composition for forming a pressure-sensitive adhesive layer provided in a pressure-sensitive adhesive sheet for these applications (particularly, an application for directly attaching to a metal surface) is preferably in a constitution enabling the formation of a pressure-sensitive adhesive layer having, as a base polymer, an acrylic copolymer having a copolymerization composition containing no carboxyl group-containing monomer. As the background art documents regarding these kinds of technologies, JP-A-2007-63536, JP-A-2005-325250 and JP-A-2007-264092 may be exemplified. As the background art documents regarding the acrylic adhesive, JP-A-2007-264092 may be exemplified. JP-A-2003-165965 is a background art document regarding a pressure-sensitive adhesive and an adhesive.
However, a pressure-sensitive adhesive composition containing no carboxyl group-containing monomer tends to have insufficient adhesion characteristics of a pressure-sensitive adhesive sheet formed from the composition. Regarding this, JP-A-2007-63536 describes copolymerization with a specific maleimide-based compound, JP-A-2005-325250 describes copolymerization with a nitrogen atom-containing monomers such as N-acryloylmorpholine, and JP-A-2000-303045 describes use of specific monomers such as phenoxyethyl acrylate, as main monomer components. However, even in the pressure-sensitive adhesive sheets formed by using these technologies, there is room for improvement in adhesive performance. For example, it would be useful to provide a pressure-sensitive adhesive composition that exhibits a desired cohesive force without the use of a carboxyl group-containing monomer, and further is capable of forming a pressure-sensitive adhesive sheet having increased repelling resistance (adhesiveness on a curved surface).